This invention relates to a bicycle, more particularly to a bicycle with two speed-changing sprocket-and-chain units.
Referring to FIG. 1, a know bicycle 100 includes a front wheel 11, a rear wheel 12, a large, driving sprocket 13 with a crank 16, a small, driven sprocket or freewheel 14 and a chain 15 trained on the driving sprocket 13 and the driven sprocket 14. Generally, the largest driving sprocket 13 has 54 sprocket teeth, while the smallest driven sprocket 14 has 12 sprocket teeth. When the driving sprocket 13 rotates one revolution, the driven sprocket 14 rotates 4.5 revolutions. The speed ratio of 54/12 is sufficiently large, but wastes the rider's energy.
Referring to FIG. 2, in a case where the force applied to the pedal is F1 and the length of the crank is r1, the produced moment is (M). When the crank sprocket with a radius r2 is driven by the moment (M), the force created on the chain is F2. This can be expressed by the following formula: EQU F1.times.r1=M=F2.times.r2
the greater the radius r2, the greater the force F1 needed to create the same force F2, resulting in wasting more energy when propelling the bicycle. As shown in FIG. 3, a speed-changing mechanism 20 can be provided between the driving sprocket and the driven sprocket. The driving sprocket includes two sprocket members of different diameters, while the driven sprocket includes five or six sprocket members of different diameters. However, because the sprocket members of the driven sprocket do not relate to the features of this invention, further description thereof will be omitted. Although the chain 21 can be moved to engage with the teeth of a diameter-reduced driving sprocket 22 to save the rider's energy, the speed ratio of the driven sprocket 23 to the driving sprocket 22 is decreased.